1. Field of the Invention
The present invention relates to wireless communication and, more particularly, to a method of a relay station transmitting and receiving a signal and an apparatus for performing the same.
2. Related Art
An IEEE (Institute of Electrical and Electronics Engineers) 802.16e standard is a sixth standard for IMT (International Mobile Telecommunication)-2000 in the ITU-R (ITU-Radio communication Sector) under the control of ITU (International Telecommunication Union) in the year 2007 and has been adopted by the name of ‘WMAN-OFDMA TDD’. The ITU-R is preparing for an IMT-Advanced system which is the next-generation 4G mobile communication standard since the IMT-2000. IEEE 802.16 WG (Working Group) has decided to go ahead with an IEEE 802.16m project with the object of writing an amendment standard of the existing IEEE 802.16e which is a standard for the IMT-Advanced system at the end of the year 2006. As can be seen from the above object, the IEEE 802.16m standard involves the two aspects of the past continuation (i.e., an amendment of the IEEE 802.16e standard) and the future continuation (i.e., a standard for the next-generation IMT-Advanced system). Accordingly, the IEEE 802.16m standard is required to fulfill all advanced requirements for the IMT-Advanced system while maintaining compatibility with a Mobile WiMAX system based on the IEEE 802.16e standard.
In the case of a wideband wireless communication system, an effective transmission and reception scheme and a utilization scheme have been proposed in order to maximize the efficiency of limited radio resources. One of the systems taken into consideration in the next-generation wireless communication system is an Orthogonal Frequency Division Multiplexing (OFDM) system which can attenuate an Inter-Symbol Interference (ISI) effect with low complexity. The OFDM system converts serially inputted data symbols into N number of parallel data symbols and carries them on N number of separated subcarriers. The subcarriers maintain orthogonality in the frequency domain. Each of the orthogonal channels experiences independent frequency selective fading. Accordingly, complexity at a receiving terminal can be reduced, the interval between transmitted symbols is lengthened, and ISI can be minimized.
Orthogonal Frequency Division Multiple Access (OFDMA) refers to a multi-access method for realizing multi-access by independently providing some of available subcarriers to respective users in a system using OFDM as a modulation method. In the OFDMA method, it is common that frequency resources called the subcarriers are provided to respective users, and the frequency resources do not overlap with each other because they are independently provided to the plurality of users. Consequently, the frequency resources are exclusively allocated to the users. In the OFDMA system, frequency diversity for multiple users can be obtained through frequency selective scheduling, and subcarriers can be allocated in various ways according to a permutation method for the subcarriers. Further, the efficiency of a spatial region can be increased through a spatial multiplexing scheme using multiple antennas.
Meanwhile, a wireless communication system including relay stations is being developed. The relay station (RS) functions to expand the cell coverage and improve the performance of transmission. If a base station provides services to a mobile station, placed at the boundary of the coverage of the base station, through a relay station, the cell coverage can be expanded. Furthermore, if a relay station improves reliability in the transmission of signals between a base station and a mobile station, the amount of transmission data can be increased. Although a mobile station is placed within the cell coverage of a base station, it may use a relay station if the mobile station is placed in the shadow region.
The relay station can be chiefly divided into two types. The first type is a transparent relay station. In this type, a base station determines all pieces of information necessary for a relay process, and the transparent relay station simply relays data, received from the base station, to a subordinate relay station or a mobile station. The transparent relay station uses the same carrier frequency as a superordinate or subordinate station. The second type is a non-transparent relay station. The non-transparent relay station directly performs resource allocation necessary for a relay process, the determination of a Modulation and Coding Scheme (MCS) level, power control, etc. and relays data. The non-transparent relay station may use the same carrier frequency as a superordinate or subordinate station or may use a different carrier frequency from the superordinate or subordinate station.
In a centralized scheduling mode, a base station determines the allocation of frequency bands for a relay station (RS) and a relay station mobile station (RS-MS). In a distributed scheduling mode, a RS determines the allocation of frequency bands to a RS-MS while operating in conjunction with a base station. A transparent base station can be operated only in the centralized scheduling mode, and a non-transparent base station can be operated in the centralized or distributed scheduling mode.
Amplify and Forward (AF) and Decode and Forward (DF) can be used as a relay method for a RS. In the AF method, the RS amplifies data received from a base station and transmits the data to a mobile station (MS). In the DF method, the RS checks a destination station by decoding data received from a base station, encodes the decoded data, and relays the encoded data to a subordinate RS or a MS (i.e., the destination station).
In a wireless communication system including such a relay station, a new frame structure different from a conventional frame structure is required. The RS can use the same frequency band, used to transmit a signal to a base station, as a frequency band used to receive a signal from a RS-MS. Alternatively, the RS can use the same frequency band, used to receive a signal from a base station, as a frequency band used to transmit a signal to a RS-MS. The RS is difficult to transmit and receive a signal at the same time in the same frequency band because of self-interference. Accordingly, the time for switching the operation mode between the transmission and reception of a signal is required. It is assumed that in the operation mode transition time, a RS is in general unable to transmit or receive a signal.
There is a propagation delay time that must be taken into consideration along with the operation mode transition time. The propagation delay time can be considered as a physical transfer time which is taken to transmit and receive a radio signal between two communication stations. In other words, in a wireless communication system including a relay station, the RS has to communicate with a base station and a MS on the basis of a timing relationship in which the operation mode transition time, the propagation delay time, and so on are taken into consideration.